Mechnical+engineering+interview+Questions+and+Answers

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Frequently Asked Mechanical Engineering Interview Questions and Answers

1) What is the importance of the Thermodynamics in the field of Mechanical Engineering?

All the mechanical engineering systems are studied with the help of thermodynamics. Hence

it is very important for the mechanical engineers.

2) How many Laws of Thermodynamics are there?

There are three laws of the thermodynamics.

First Law: Energy can be neither created nor destroyed. It can only change forms.In any

process in an isolated system, the total energy remains the same.

Second Law: When two isolated systems in separate but nearby regions of space, each in

thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at

some time allowed to interact, breaking the isolation that separates the two systems, and they

exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium.

The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of

the final exchanging systems. In the process of reaching a new thermodynamic equilibrium,

entropy has increased, or at least has not decreased.

Third Law: As temperature approaches absolute zero, the entropy of a system approaches a

minimum.

3) State Laws of conservation of energy?

According to the laws of conservation of energy, “energy can neither be created nor be

destroyed. It can only be transformed from one form to another.”

4) Is the boiler a closed system?

Yes definitely the boiler is a closed system.



5) What is Carnot engine?

It was being designed by Carnot and let me tell you that Carnot engine is an imaginary engine

which follows the Carnot cycle and provides 100% efficiency.

6) Which formula forms a link between the Thermodynamics and Electro chemistry?

Gibbs Helmholtz formula is the formula which forms the link between the thermodynamics

and electromagnetism.

∆Hs/R = [∂ lnp /∂ (1/T)]x

where: x – mole fraction of CO2 in the liquid phase

p – CO2 partial pressure (kPa)

T – temperature (K)

R – universal gas constant

α – mole ratio in the liquid phase (mole CO2 per mole of amine)

7) Which is the hardest compound known?

Diamond.

8) What is Hess Law?

According to the Hess law the energy transfer is simply independent of the path being

followed. If the reactant and the product of the whole process are the same then same amount

of energy will be dissipated or absorbed.

9. Which has more efficiency: Diesel engine or Petrol engines?

Off course Diesel engine has the better efficiency out of two.

9) What is the difference between Critical Speed and Whirling Speed?

In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical angular

velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or

gear. As the speed of rotation approaches the objects natural frequency, the object begins to

resonate which dramatically increases system vibration. The resulting resonance occurs

regardless of orientation.Whirling Speed is due to the unbalanced forces acting on a rotating

shaft.

10) How a Diesel Engine Works as Generator?

Diesel engine is a prime mover, for a generator, pump,and for vehicles etc. generator is

connected to engine by shaft. mostly in thermal power plat ,there is an engine is used to drive

generator to generate power.

11) Explain Second Law of Thermodynamics?

The entropy of the universe increases over time and moves towards a maximum value.

12) Compare Brayton Cycle and Otto Cycle?

The heat addition and rejection processes in Otto cycle are of constant volume, whereas in

Brayton cycle, they are of constant pressure.



-Otto cycle is the ideal cycle for spark ignition engines.

-Brayton cycle is the ideal cycle for gas power turbines.

13) What is the purpose of Scrapper Ring?

Scrap the excess lube oil from the cylinder walls. there by preventing oil from entering

combustion zone.

14) What is DTSI Technology?

DTSI stands for Digital Twin Spark Plug Ignition. The vehicles with DTSI Technology use 2

spark plugs which are controlled by digital circuit. It results in efficient combustion of air fuel

mixture.

• Digital - Since the spark generation will be initiated by a microchip.

• Twin - Since two spark plugs will be used.

• Spark ignition - Since the ignition will be done via a spark.

15) How to Find, Ductile-Brittle Transition Temperature in Metals?

The point at which the fracture energy passes below a pre-determined point for a standard

Impact tests. DBTT is important since, once a material is cooled below the DBTT, it has a

much greater tendency to shatter on impact instead of bending or deforming.

16) What is the importance of Thermodynamics?

All the mechanical engineering systems are studied with the help of thermodynamics. Hence

it is very important for the mechanical engineers.

17) What is the difference between P11 and P12 Pipes?

P11 the chromium molybdenum composition that is 1% ofchromium and 1/4% of

molybdenum

P12 the chromium molybdenum composition that is 1% ofchromium and 2% of molybdenum

18) State difference between AnitiFriction Bearing and Journal Bearing?

Generally, journal bearings have higher friction force, consume higher energy and release

more heat, but they have larger contact surface, so normally used in low speed high load

applications. In anti friction bearings friction is less. One object just rolls over each other.

19) What is the difference between Technology and Engineering?

Engineering is application of science. Technology shows various methods of Engineering. A

bridge can be made by using beams to bear the load,by an arc or by hanging in a cable; all

shows different technology but comes under civil engineering and science applied is laws of

force/load distribution.

20) How to Measure Temperature in Wet Bulb Thermometer?



Wet bulb temperature is measured in a wet bulb thermometer by covering the bulb with a

wick and wetting it with water. It corresponds to the dew point temperature and relative

humidity.

21) What are the Advantages and Disadvantages of using LPG in Car?

Advantages

1. Complete combustion

2. Fuel saving

3. Homogenous combustion

Disadvantages

1. As complete combustion is occurring ,more heat liberated,not advised for long journey,

engine will be over heated

2. Installation is difficult

3. Reduce engine life efficiency

22) What is the difference between Speed and Economic Speed?

The rated speed tells us about the maximum speed which can be achieved by a vehicle or

some other machine but the economical speed means the speed limit at which the machine

works efficiently with least consumption of fuel.eg-in normal bikes(not racing),the

max.speed limit shown on speedometer is upto 120 kmph but companies always advice their

customers to drive such bikes at around 60 kmph to have maximum mileage.

23) What is Powder Technology?

Powder technology is one of the ways of making bearing material. In this method metals like

bronze, Al, Fe are mixed and compressed to make an alloy.

24) State all the laws of Thermodynamics?

There are three laws of the thermodynamics.

First Law: Energy can be neither created nor destroyed. It can only change forms. In any

process in an isolated system, the total energy remains the same.

Second Law: When two isolated systems in separate but nearby regions of space, each in

thermodynamic equilibrium in itself, but not in equilibrium with each other at first, are at

some time allowed to interact, breaking the isolation that separates the two systems, and they

exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium.

The sum of the entropies of the initial, isolated systems is less than or equal to the entropy of

the final exchanging systems. In the process of reaching a new thermodynamic equilibrium,

entropy has increased, or at least has not decreased.

Third Law: As temperature approaches absolute zero, the entropy of a system approaches a

minimum.

25) State the difference between Unilateral and Bilateral Tolerance?

A unilateral tolerance is tolerance in which variation is permitted only in one direction from

the specified direction.e.g. 1800 +0.000/-0.060

Bilateral tolerance is tolerance in which variation is permitted in both direction from the

specified direction.e.g. 1800 +0.060/-0.060



26) What is the abbreviation of welding rod 7018?

7018 =

70=tensile strength 70000psi

1= welding position

8=current flux

27) What is difference between Welding and Brazing?

In Welding concentrated heat (high temperature) is applied at the joint of metal and fuse

together.

In Brazing involves significantly lower temperatures and does not entail the melting of base

metals. Instead, a filler metal is melted and forced to flow into the joint through capillary

action.

28) Which has more Efficiency Diesel Engine or Petrol Engine?

Diesel engine has the better efficiency out of two.

29) What is Difference between stamina and strength?

Strength is capability over a short length of time and Stamina is the ability to keep going

continuously.

30) What is Hydrostatic System?

Hydrostatics is the study of fluid bodies that are

• At rest

• Moving sufficiently slowly so there is no relative motion between adjacent parts of

the body

For hydrostatic situations

• There are no shear stresses

• There are only pressure forces that act perpendicular to any surface.

It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies

energy to motor and motor gives return energy to pump supply.

31) What is Cotter joint?

A cotter joint is used to connect rigidly two co-axial rods or bars which are subjected to axial

tensile or compressive forces. Here shaft is locked in place by a smaller pin that passes

through the side of the lug and partly or completely through the shaft itself. This locking pin

is named as cotter.

32) How is the excess discharge pressure prevented?



Ans. Discharge pressure prevented by a pressurized spike cushion. Here the system

employs a pressurized cushion of air and a two o-ring piston, which permanently separates

this air cushion from the water system. When the valve closes and the water flow is suddenly

stopped, the pressure spike pushes the piston up the arrester chamber against the pressurized

cushion of air. The air cushion in the arrester reacts instantly, absorbing the pressure spike

that causes water hammer.

33) What is the difference between Strainer and Fitler?

Strainer for coarse size, Filter is more accurate than Strainer.

34) What is the position of Piston Ring?

Ans. In 180 degree angle the Top ring, Second ring and Oil ring are fixed. Position the ring

approximately 1 inch gap below the neck.

35) Why Deareator are placed at Hieght, In Thermal Power Plant?

To build a Very high pressure and the temperature for a boiler feed water pump and it

discharge high pressure water to the boiler.

And to provide the required Net Positive Suction Head (NPSH) for the BFW pump and to

serve as a storage tank to ensure a continuous supply of feed water during rapid changes in

BFP.

36) What is meanst by One Tonn Air-Conditioner?

1 ton refrigeration means 210 kJ/min extracts heat from thesystem.

37) State 1st Law of Thermodynamics?

HEAT AND MECHANICAL WORK ARE MUTUALLY CONVERTABLE. ENERGY

CAN BE CREATED NOR BE DISTROYED BUT IT CAN BETRANSFERED FROM ONE

FORM TO ANOTHER FORM.

38) If you heat a steel pipe with the hole at center, does heat affects the hole diameter?

It gets bigger.

39) Why the Centrifugal Pump is called High Discharge pump?

Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push

out the fluid. So the liquid entering the pump receives kinetic energy from the rotating

impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity,

transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high

rate. It is given in the following formulae:

Centrifugal force F= (M*V2)/R.

Where,

M-Mass

V-Velocity

R-Radius

40) How Cavitation can be eliminated by Pump?



Cavitation means bubbles are forming in the liquid.

· To avoid Cavitation, we have to increase the Pump size to One or Two Inch;

To increase the pressure of the Suction Head, or

· Decrease the Pump Speed.

41) Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?

The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow

velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller

than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate.

So pressure drop happened between pump suction and the vanes of the impeller. Here air

bubbles or cavities are formed because of liquid vapour due to increase in temperature in

impeller. This air bubbles are transmitted to pump which forms cavitation.

42) Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is

smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

43) Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?

The centrifugal has varying flow depending on pressure or head, whereas the Positive

Displacement pump has more or less constant flow regardless of pressure.

Likewise viscosity is constant for positive displacement pump where centrifugal pump have

up and down value because the higher viscosity liquids fill the clearances of the pump

causing a higher volumetric efficiency. When there is a viscosity change in supply there is

also greater loss in the system. This means change in pump flow affected by the pressure

change.

One more example is, positive displacement pump has more or less constant efficiency,

where centrifugal pump has varying efficiency rate.

44) Why Entropy decreases with increase in temperature?

ds=dQ/T Entropy is inversely proportional to the temperature so, as temp. Increases, entropy

decreases.

45) Why different types of sound are produced in different bikes, though they run on SI

Engines?

Engine specifications are different in different manufactures like as Bore Diameter (CC),

Ignition timing. Also the exhaust passage takes more responsible for sound.

46) How much Watt means 1Hp?

746.2 Watt

47) Explain Bicycle Rear Wheel Sprocket working?

Rear wheel sprocket works under the principle of ratchet and pawl.



48) Definition of Octane Number and Cetane Number?

Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the

mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the

mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

49) Which Mechanism is used in Automobile gearing System?

Differential mechanism

50) When Crude Oil is Heated, Which Hydro Carbon comes first?

Natural gas (Gasoline)… at 20 Celsius

51) How to calculate Bearing number ti Diameter of the inner and outer?

Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according

to type of load we have to choose the type of bearing and that will give prior number of the

bearing.

52) The Fatigue life of a part can be improved by?

Improving the surface finish by Polishing & providing residual stress by Shot peening.

53) What happens if gasoline is used in a Diesel Engine, Siesel Engine will work?

No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel

engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may

blast.

54) Poissons Ratio is Higher in, Rubber/Steel/Wood?

When a material is compressed in one direction, it usuallytends to expand in the other two

directions perpendicular tothe direction of compression. This phenomenon is called

thePoisson effect. Poisson’s ratio is a measure of the Poisson effect.

For rubber = 0.5

For steel = 0.288

For wood < 0.2

Thus Poisson’s ratio is higher in RUBBER.

55) What happens if gasoline is used in a Diesel engine?Diesel engine will work ?

No, It will not work,as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel

engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may

blast.

57) Why different types of sound are produced in different bikes though they say run on SI

engine

Engine specifications are different in different manufactures like as Bore Diameter(CC),

Ignition timing.Also the exhaust passage take more responsible for sound.



58) Why entropy decreases with the increase in temperature?

ds=dQ/TEntropy is inversely proportional to the temperature so.as temp. increases,entropy

decreases.

59) 1 hp how much watt?

746.2Watt

60) How to calculate bearing number to diameter of the inner and outer

Divide the shaft diameter size by 5, it will give last two digitof the bearing no. and according

to type of load we have tochose the type of bearing and that will give prior no. ofthe bearing.

61) Explain Bicycle rear wheel Sprocket working?

Rear wheel sprocket works under the principle of ratchet and pawl.

62) Definition of Octane number & Cetane number

Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the

mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the

mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

Poisons ratio is higher in (rubber/steel/wood)

63) What is important to remember about radial bearings?

A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder.

These bearings must be accurately aligned to maintain the close clearance between the shaft

and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the

he rotor, great harm can be done to the turbine.

64) How many governors are needed for safe turbine operation? Why?

Two independent governors are needed for safe turbine operation. One is an over speed or

emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).

The second, or main governor, usually controls speed at a constant rate; however, many

applications have variable speed control.

65) How is a fly ball governor used with a hydraulic control?

As the turbine speeds up, the weights are moved outward by centrifugal force, causing

linkage to open a pilot valve that admits and releases oil on either side of a piston or on one

side of a spring-loaded piston. The movement of the piston controls the steam valves.

66) What is meant by critical speed?

It is the speed at which the machine vibrates most violently. It is due to many causes, such as

imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the



turbine should be hurried through the known critical speed as rapidly as possible. (Caution,

be sure the vibration is caused by critical speed and not by some other trouble).

67) How is oil pressure maintained when starting or stopping a medium-sized turbine?

An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by

a hand crank; others are motor-driven. This pump is used when the integral pump is running

too slowly to provide pressure, as when starting or securing a medium-sized turbine.

68) Besides lubrication, what are two functions of lubricating oil in some turbines?

In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in

some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

69) Do you stop cooling-water flow through a steam condenser as soon as the turbine is

slopped?

You should keep the cooling water circulating for about 15 mill or more so that the condenser

has a chance to cool down gradually and evenly. Be sure to have cooling water flowing

through the condenser before starting up in order to prevent live steam from entering the

condenser unless it is cooled. Overheating can cause severe leaks and other headaches.

70) How can the fatigue damage on high-pressure blades be corrected?

Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc

admission. This can be corrected by switching over to full arc admission technique.

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